1. Field of the Invention
This invention relates to multifaceted mirror laser printing systems. More particularly, this invention relates to reducing the average data rate used in such a system in an uncomplicated and cost effective manner.
2. Description of the Prior Art
The Minnesota Mining & Manufacturing Company P831 Laser Imager, which is described in U.S. Pat. No. 4,583,128, is an example of a polygon based mirror laser printing system. Such systems create a hard copy of an image by exposing a photosensitive medium to a laser beam, the intensity of which is determined by digital data which is input to the printing system from a host device at an input digital data rate. The image is created by exposing the photosensitive medium, in a pixel-by-pixel (point-by-point) manner, to the laser beam which is modulated in intensity according to the input digital data. The input digital data causes the intensity of the beam to be modulated at each pixel (point) as it is scanned across the photosensitive medium in a raster fashion. The photosensitive medium is moved in a longitudinal direction while the beam is being scanned in a transverse direction. Each time the beam is scanned across the medium it produces a scan line. The image to be printed is comprised of multiple (nominally 300) scan lines per inch in the longitudinal direction and multiple (nominally 300) pixels per inch in the transverse direction.
Generally, to achieve the highest quality output imaging, the entire digital image must be printed without stopping. Therefore, it is desirable to transport the photosensitive medium in the longitudinal direction in a smooth, continuous manner rather than in an incremental, step type of manner. With continuous transport, the medium is not stopped once printing has begun. The digital data, therefore, must be supplied to the laser printer by the host device fast enough so that there is no loss of image quality due to loss of synchronization between the input digital data rate from the host device and the data rate requirements of the laser printer. The speed at which the host device must transfer digital data to the printer is determined by the product of the number of pixels per line and the number of lines printed per second.
For a high speed laser printer, it is very common that the necessary output speed exceeds the highest speed at which the host device can transfer digital data to the printer. For example, the P831 Laser Imager requires data to be transferred to the laser at a speed of approximately 2.5 million pixels per second (averaged over one scan line). This value usually exceeds the data transfer capabilities of a host device which typically may be less than 1 million pixels per second. Therefore, a problem which must be addressed is how to match the input data rate requirements of the laser printer with the transfer speed constraints of the host device.
In the past, this problem has been addressed by the inclusion of a mass memory system within, or closely connected to, the laser printer. This memory is loaded with the digital data, representing the entire image to be printed, from the host device before any printing begins. Therefore, no data transfer constraints are placed on the host device. Recent memory technology and design have allowed the digital data stored in the memory to be read out at a rate which matches the data rate requirements of the laser printer. For example, the P831 Laser Imager employs up to 20 megabytes of RAM which are used to temporarily store a page of image data prior to printing.
There are several disadvantages to this approach of matching the data rates. First, the amount of digital data required to represent a complex, high resolution, digital image can be very large and, although magnetic disk memory can sometimes be used as an alternative to integrated circuit memory, very fast access time memory is required. Due to the size and access time required of this memory, it is very costly.
Second, such a system entails a significant duplication of resources. If the host device and the laser printer are viewed as a complete system, the memory which is added to the laser printer may be duplicating a memory which already exists in the host device.
Third, to guarantee high speed operation of such a large semiconductor memory, special techniques are required in implementation. These techniques are very complex and costly.